1. Field of the Invention
This invention relates to a vacuum pump in which the pressure at that outlet port is maintained at atmospheric pressure, and more particularly to a shaft seal structure between pump stages of such a pump and a driving means for turning impellers in pump stages thereof.
2. Description of the Prior Art
A semiconductor manufacturing apparatus requires a high-degree vacuum chamber, which consists generally of a turbomolecular pump having excellent pumping performance in a molecular flow region thereof. However, the property of the turbomolecular pump depends upon the pressure at the outlet port. For example, in order to obtain normal performance of this pump, a pressure of 10.sup.-2 -10.sup.3 Torr at the outlet port is required. In order to obtain such a low pressure at the outlet port, it is necessary that a pre-treatment be done by an auxiliary pump, which is usually a rotary pump. Although the turbomolecular pump is used advantageously for obtaining high vacuum, it requires an auxiliary pump. This causes the dimensions of the apparatus to increase, and the operation thereof to become complicated. Therefore, the use of the turbomolecular pump is limited.
Under the circumstances, it has been demanded that a vacuum pump of unitary construction, which is made on the basis of the principle of a turbomolecular pump in which the characteristics of other types of pumps are incorporated, be practically produced. The vacuum pumps of unitary construction include the vacuum pump disclosed in U.S. Pat. No. 3,969,039. This vacuum pump is constructed so that it is provided in the portion of the interior of its casing which is between an inlet port and a outlet port thereof with axial-flow turbomolecular pump stages, and drag molecular pump stages, centrifugal compressor stages and vortex diode pump stages which pump stages are connected in the mentioned order.
In a conventional vacuum pump, it is impossible that a driving means, such as a motor for rotating a rotor in a pump unit and an outlet port of the pump, i.e. a hollow in a pump unit are shut off from each other perfectly. Consequently, a corrosive gas and a dust-containing gas, which are sucked by the vacuum pump, and which are noxious to the driving means, enter the driving means to cause the parts thereof to be corroded or damaged by the dust. Also, the mist of a lubricating oil supplied to the bearings would enter the interior of the pump to hamper the cleaning of the same.
The means for solving these problems include a means for filling the interior of the driving means with a purge gas.
In the above-described example, a purge gas is supplied constantly to the interior of the driving means to prevent the entry of noxious gases from the hollow space in the pump unit thereinto. In order that the purge gas is discharged from the interior of the driving means into the hollow space in the pump unit as the presence of the purge gas continuously overcomes that in this hollow space, a large quantity of purge gas is required, and this causes the operation cost to increase.
In the above example, a spiral groove is formed in the wall which defines a clearance through which the purge gas in the interior of the driving means is discharged to the hollow space in the pump unit, in such a manner the purge gas is discharged from the driving means to this hollow space as the spiral groove is rotated. However, even in this case, an increase in the quantity of pure gas in use cannot be avoided.